My dissertation defends two controversial and seemingly incompatible theses, both drawn from the moral philosophy of Bernard Williams. The first is that a type of ethical knowledge is possible. The second is that such knowledge can be undermined by certain forms of critical reflection. Both of these claims are tied closely to the possibility of there being (what are sometimes called) ?thick? ethical concepts: ethical concepts whose application is guided by users? perceptions of acts and situations, and which can also guide their actions, by providing them with practical reasons of various sorts. The knowledge that comes with the disposition to actively use a thick concept is that certain defeasible practical reasons follow from the facts of a situation which determine whether such a concept should apply.